Thanks Jess!!
Yes it will happen automatically, tax due is calculated cumulatively across the year so once your pay drops they reconsider your earnings for the full year and start issuing refunds.
For me, I got a letter and you had to go online and actually claim it back!
My workplace started paying mine back to me when I went down to stat pay which was helpful as it topped it up!
I think it should be automatically reimbursed, towards the end of the year I started getting small refunds in my wage packet because I only had 6 weeks full pay. You should get one after your P60 but if not then you need to contact HMRC!